1. Field of the Invention
The present invention relates to a lens frame mounted in an image projection equipment such as projection television and more particularly to a lens frame in which a lens barrel is rotated to be moved in the direction of lens light axis to adjust a focus of the lens when projecting a CRT image or a liquid crystal display image on a screen.
2. Prior Art
FIG. 6 shows an image projection apparatus in the projection television of this kind. In the Figure, a lens barrel 10 which has a built-in projection lens is supported on a lens barrel support 20, which is mounted on a projection tube 40 through an optical coupler 30.
An image formed by the projection tube 40 is projected onto a screen 50 by a projection lens contained in the lens barrel 10.
FIGS. 7 to 9 show conventional lens frames of this kind, which are described below.
The lens barrel 10 shaped like a circular cylinder is fitted in a cylinder portion 20a of the lens barrel support 20. The cylinder portion 20a is formed with a groove 20b which is inclined with respect to the circumferential direction and through which the lens barrel 10 is attached with a fixing portion 60. The fixing portion 60, as shown in FIG. 8, has a screw 60a screwed through a thumbnut 60b, a washer 60c and a collar into a projection 10a formed on the side of the lens barrel 10, with the collar 60d fitted in the groove 20b of the cylinder portion 20a.
When the lens barrel 10 is rotated relative to the lens barrel support 20 and the collar 60d is slid along the groove 20b, the lens barrel 10 is moved along the axis of the cylinder to adjust the focus on the screen.
With the lens in focus, the thumbnut 60b is turned to press the lens barrel 10 against the cylinder portion 20a of the lens barrel support 20, thus fixing the lens barrel 10 to the lens barrel support 20.
In such a lens frame, a guide portion for guiding the lens barrel 10 in a longitudinal direction is the groove 20. When the lens barrel support 20 is to be formed by a plastic projection molding, the groove 20b constitutes an undercut and cannot be formed by upper and lower molding dies alone, requiring a slide core for making the groove 20b. This makes the structure of the dies complex, which in turn increases the cost and makes the maintenance complex.
Furthermore, the fixing portion 60 has a number of parts. Assembly of the lens barrel 10 and the lens barrel support 20 requires many steps, i.e., aligning the projection 10a of the lens barrel 10 with the groove 20b in the cylinder portion 20a, fitting the thumbnut 60b, the washer 60c and the collar over the screw 60a, and driving the screw 60a into the projection 10a of the lens barrel 10.
Another problem is that if the axis of the lens barrel 10 and the cylinder portion 20a are aligned as shown in FIG. 9a before they are fastened together by the thumbnut 60b, the turning and tightening of the thumbnut causes the lens barrel 10 to be pressed against the inner surface of the cylinder portion 20a at the fixing portion 60, shifting the axis of the lens barrel 10 out of alignment with the axis of the cylinder portion 20a, as shown in FIG. 9b.
As a result, the center axis of the lens barrel support 20, which is fixed in a specified optical position relative to the coupler 30, becomes deviated from the center axis (lens optical axis) of the lens barrel 10, bringing the image out of focus.